Video conferencing systems allow for simultaneous exchange of audio, video and data information among multiple conferencing sites. Systems known as multipoint control units (MCUs) perform switching functions to allow three or more sites to intercommunicate in a conference. The MCU links multiple video conferencing sites together by receiving frames of digital signals from the sites, processing the received signals, and retransmitting the processed signals to appropriate sites. The digital signals include audio, video, data and control information. In a switched conference, the video signal from one of the conference sites, typically that of the loudest speaker, is broadcast to each of the participants. In a continuous presence conference, video signals from two or more sites are spatially mixed to form a composite video signal for viewing by conference participants.
In a typical continuous presence conference, the video display is divided into quadrants and up to five sites are selected at conference setup from the sites in the conference for display in the quadrants. Up to five combination organized into the four quadrants, are created statically and are fixed for the conference. Each combination is referred to as a video mix and each selected site views a video mix comprising the other four sites such that a selected site never receives its own video signal. One of the video mixes is also selected as a broadcast mix for viewing by those remaining sites in the conference that are not among the five selected sites.
In the typical continuous presence conference, no attempt is made to associate any particular site with a particular display quadrant. That is, the quadrant organization used does not provide for a site to appear in the same quadrant in each of the five video mixes. Since the five selected sites are chosen at conference setup, this selection remains static throughout the conference, unless a particular selected site leaves the conference, in which case one of the unselected sites may be selected as a replacement.
In another conference system, it is possible to display video information from an unselected site in a single video mix that is broadcast to all sites in the conference. However, since only one broadcast video mix is provided, each of the four sites in the broadcast mix sees its own video.